This invention relates to frequency modulation ("FM") transmission, and particularly to FM transmission of signals representative of visual images and sound.
In FM transmission, the instantaneous frequency, f, of a carrier wave is made proportional to an information signal, x(t), so that the signal in the channel, y(t), is given by: EQU y=cos 2.pi.ft=cos 2.pi.(f.sub.0 +mx)t,
where m, the modulation index, is the proportionality factor between the input signal and the instantaneous frequency and f.sub.0 is the unmodulated carrier frequency. FM has two characteristics that make it desirable for transmission, and particularly for transmission of video signals in certain cases. One is its ability to exchange bandwidth and signal-to-noise ratio (SNR), i.e., by making the channel bandwidth higher than the input signal bandwidth, the received SNR can be made higher than the channel carrier-to-noise ratio (CNR). This is very useful in satellite transmission. The other is that, since the receiver is sensitive primarily to the frequency of the modulated signal, it is relatively insensitive to amplitude variations such as may occur due to "drop-outs" in magnetic recording.
The relationship between the input signal and the spectrum of the rf (radio frequency) signal in FM is quite complex, but, broadly speaking, the bandwidth of the rf signal is proportional both to the amplitude and frequency of the input. Thus in areas of an image having large-amplitude, fine detail, the rf bandwidth is large, while in relatively blank areas, the rf bandwidth is small. As it happens, noise in the received signal, from whatever cause, is most noticeable in the blank areas and least noticeable in the highly detailed, or "busy " areas. In fact, if noise is of a level as to be moderately annoying in the blank areas, it is completely invisible in the detailed areas.
Because of the relationship between the input signal and the rf bandwidth, the choice of modulation index in conventional systems is a tradeoff between noise visibility in relatively blank areas and distortion in highly detailed, high-contrast areas caused by truncation of the rf spectrum due to the fixed channel bandwidth. It is best not to choose the index so as absolutely to eliminate all distortion, since this would needlessly sacrifice SNR in blank areas. Some distortion of extreme slopes and/or amplitudes in the video, such as caused by specular highlights, is preferable.
Several methods have been used in partially successful attempts to improve the tradeoff discussed above. A linear method (Hirota U.S. Pat. No. 4,607,285) pre-emphasizes the spatial high-frequency components, both vertical and horizontal, before modulation and de-emphasizes them afterward. Several nonlinear methods have been used in which high-amplitude transients are emphasized less than low-amplitude transients, permitting a higher degree of pre-emphasis of the low-level detail (Hirota U.S. Pat. No. 4,618,893). In another nonlinear method, the amount of pre-emphasis is made to depend on the level of the (pre-emphasized) signal, and in still another (van Cang U.S. Pat. No. 4,007,483), the high-frequency components, principally the color subcarrier and its sidebands, are subjected to a nonlinear compression before modulation and a nonlinear expansion afterwards (the entire process is called static companding) so that some noise is shifted from areas of low-level detail to areas of high level detail. Some of these processes may also be combined.
In the paper, "A Two-Channel Picture Coding System II: Adaptive Companding and Color Coding," IEEE Transactions on Communications, Vol. COM-29, No. 12, December 1981, by W. F. Schreiber and R. R. Buckley, a system for reducing the visibility of quantization noise is described. In that system, the image is divided into small blocks, typically 3.times.3 to 8.times.8 picture elements (samples or "pels"). In each block, a value of a gain factor is found such that when the signal is multiplied by the factor, the multiplied signal just does not exceed the maximum permissible amplitude of the channel. At the receiver, the received signal is divided by the same factor, thus reducing the noise by the same amount in those areas where the signal, being small, can be multiplied by a factor larger than unity. To avoid block effects, the factor used at each pel is determined from the block factor by 2-dimensional interpolation, and thus the factor varies smoothly from pel to pel. The block factors are transmitted along with the signal, but they do not consume much channel capacity. The methods discussed in the paper operate independently on successive frames of a TV signal and do not take advantage of the special properties of electrical representations of moving images.